Optical communication systems are a substantial and fast-growing constituent of communication networks. The expression "optical communication system," as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, New York) c. 1993, the disclosure of which is incorporated herein by reference. Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing is frequently employed (TDM). In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal being constructed from the portions of the signals collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single waveguide, each channel being assigned a particular wavelength. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical networks.
In order to implement wavelength division multiplexing in practical optical communication systems, it is desirable to include a mechanism for monitoring system performance. To convey monitoring information, e.g., information concerning optical signal characteristics, optical device outputs, service orders, and the like, it is desirable to provide an optical channel within the optical communications system devoted to the monitoring of system performance. Previously, optical communication system designs have concentrated on the provision of service channels for conventional systems which convey their signal payload on a single wavelength. Such a system is disclosed in U.S. Pat. No. 5, 113,459. In this patent, an optical fiber transmission line is disclosed which includes units for injecting optical service signals into the line's optical fiber. Similarly, units for extracting optical service signals from the line's optical fiber are provided. In this optical system, the wavelength provided for the telecommunications signals is in the range from 1500 to 1600 nm while the wavelength of the service signal is between 1200 to 1400 nm, preferably 1300 nm. In the '459 optical system, the service channel is supplied/extracted via a connecting unit connected to the input/output of an optical coupler whose output is coupled to the input of an optical amplifier. However, erbium-doped optical fiber amplifiers, as are employed in the '459 system, do not amplify optical transmissions in the 1300 nm wavelength range. Consequently, the optical communication system of the '459 patent provides each optical amplifier with means which diverts the service signals to a path external to the amplifier where they are electrically regenerated following optical-to-electrical conversion. Such regeneration is required when the service channel must travel a substantial distance from the signal origination point since the transmission fiber has an attenuation of approximately 0.35 dB/km in the 1300 nm range. This need to bypass optical amplifiers for regeneration increases system complexity. The high attenuation in the 1300 nm wavelength range also limits the data rate at which the optical signal is launched. The '459 patent describes data rates substantially lower than 300 kbits/sec. While a low rate can transmit conventional alarm on/off indicators, this rate is inadequate for transmission of multimedia information (e.g., voice, data, and video). Additionally, the '459 patent is silent concerning wavelength division multiplexed optical communication systems.
U.S. Pat. No. 5,170,447 also describes a service channel configuration for a conventional optical communication system. In the '447 patent, an optical communication system is disclosed having a transmission signal with a 1540 nm wavelength and a service channel with a 1300 nm wavelength. Couplers are provided in front of or behind an erbium-doped fiber amplifier to couple the 1300 nm channel into the system. As shown in FIGS. 1 and 2, the 1300 nm service channel signal travels away from the doped fiber adjacent the coupler. Again, since 1300 nm signals are not amplified by erbium-doped fiber amplifiers, the 1300 nm signals are not described as passing through the optical amplifiers.
U.S. Pat. No. 5,394,265 describes a telemetry channel in a conventional erbium-doped fiber amplifier system. In the '265 patent, a telemetry signal is supplied to an optical fiber at a carrier wavelength which corresponds to the spontaneous noise peak of a doped fiber amplifier. For erbium-doped fiber amplifiers, this noise peak occurs at 1532 nm. The other signal described as being carried by the optical fiber is a data signal having a wavelength between 1545 and 1560 nm. Although the system described in the '265 patent provides a technique by which telemetry signals can pass through an optical amplifier, the use of an in-band telemetry signal, i.e., a signal at a wavelength which is amplified by erbium-doped fiber amplifiers, is disadvantageous in the event of an amplifier failure. When an erbium-doped fiber amplifier fails, e.g., through a loss of pump light to power the amplifier, the doped fiber attenuates signals in the wavelength band which, when pumped, is the gain region of the amplifier. Therefore, the telemetry signal is attenuated by the doped fiber at a time when it is most needed to report the failure of a system element.
Additionally, the use of an in-band telemetry signal wavelength reduces the number of available wavelengths for use as payload signals. While this loss of spectrum is acceptable in a conventional system, such as the single data signal system described in the '265 patent, the decrease in available spectrum is highly disadvantageous in a wavelength division multiplexed optical communication system. This is particularly true in WDM systems employing many channels, often referred to as "dense" WDM, where channel spacings are on the order of one nanometer or less.
Thus, there is a need in the art for improved wavelength division multiplexed optical communication systems whose system performance can be reliably monitored. In particular, there is a need in the art for wavelength division multiplexed optical communication systems which include one or more optical monitoring channels, especially optical monitoring channels capable of surviving the failure of optical amplifiers.